Abstract: To investigate the causes of calf diarrhea on a dairy farm in Hubei Province, fecal samples, rectal swabs, and milk samples were collected from the dairy farm. After pathogen screening, isolation, and identification, the isolated strains were further characterized using polymerase chain reaction (PCR) and multilocus sequence typing (MLST). Additionally, their pathogenicity and drug resistance were analyzed. The results showed that 4 dominant strains were isolated from 16 anal swabs, 21 fecal samples, and 12 milk samples. Following 16S rRNA and PCR identification, 3 strains were identified as Escherichia coli O157 and designated as JZ-0720. MLST analysis determined the isolates belonged to sequence type ST10. Serological and PCR identification confirmed the isolates as serotype O157:H12. The isolate JZ-0720 carried three virulence genes (XJrfbE, uspA, and stx1) and produced a toxic effect on Vero cells similar to that of O157:H7. Infection of mice with JZ-0720 caused varying degrees of pathological lesions in multiple organs. Low-dose infection induced diarrhea in mice, while high-dose infection was lethal, with a median lethal dose (LD50) of 5.36 × 108 CFU/mL. The isolate JZ-0720 carried five drug resistance genes (gyrA, bla-CTX, bla-TEM, bla-NDM, and aadA1) and exhibited resistance to neomycin and erythromycin. The double-disk synergy test indicated the strain produced metallo-β-lactamases (MBLs) but not extended-spectrum β-lactamases (ESBLs). The traditional Chinese medicines Mume Fructus and Artemisiae Argyi Folium demonstrated significant inhibitory effects against JZ-0720. The results from this research provide a basis for the prevention and control of non-H7 serotypes of E. coli O157.

Key words: Escherichia coli O157:H12; isolation and identification; pathogenicity; drug resistance